Its been years since I have done anything in frames but as I recall, you can define which scroll bars you want to enable when you define the frame... and all you have to do is disable the horizontal scrollbar in your main frame and it should go away...

I apologize that I can't recall the command, but whatever program you are using to define the frames should give you this ability, as I recall even front page used to provide this capability...

Best Regards

Prodesk